The present invention relates generally to welding systems. More particularly, the present invention relates to systems for robotic stud arc welding of large studs without the use of a ferrule.
Stud welding is a process in which the contact surfaces of a stud, or similar fastener, and a workpiece, are heated and melted by an arc drawn between them. The stud is then plunged rapidly onto the receiving surface of the workpiece to form a weld. Arc initiation, arc time, and plunging are controlled automatically.
Two basic methods of stud welding are known as stud arc welding and capacitor discharge stud welding. Both methods usually involve direct current and arcing. A conventional stud arc welding system 10 is shown in FIG. 1. As shown, system 10 can include an electrical input 12 for incoming, three-phase power. Input 12 is connected through a fused disconnect switch 14 to a power/control unit 16. For stud arc welding, a motor-generator, a transformer-rectifier, or a storage battery can provide the power supply. The power supply for capacitor discharge stud welding is typically a low-voltage electrostatic storage system, and the arc is produced by a rapid discharge of stored electrical energy.
Control unit 16 includes a welding current controller and is coupled to a welding tool 18, which is usually a stud gun. Stud gun 18 typically includes a chuck or collet 20 into which a stud 26 can be received. In the arc method, a ceramic arc shield 22 (which is commonly known as a xe2x80x9cferrulexe2x80x9d) is generally used to shield the arc and retain the molten weld metal. Where a ferrule is used, stud gun 18 includes a ferrule holder 24 to hold ferrule 22 in place during the welding process. Control unit 16 can also be coupled to a work clamp 28 that holds workpiece 30 in place during the welding process.
In both methods, the stud serves as the electrode, while the gun is the electrode holder. Flux is generally used for stud arc welding of ferrous alloys and is an integral part of the stud. Flux provides cleaning action, arc stability, and a protective atmosphere. The arc time for capacitor discharge welding is so short that flux is not needed. Typically, in arc welding applications, a shielding or xe2x80x9cassistxe2x80x9d gas is introduced. Assist gas is typically not required with the capacitor discharge method.
Two types of stud arc welding guns that are commonly used are known as xe2x80x9cportablexe2x80x9d and xe2x80x9cfixedxe2x80x9d (i.e., production type). The principle of operation is the same for both. A portable or hand-held stud gun resembles a pistol, and is usually designed to be lightweight and durable. For example, a small gun used for welding xe2x85x9 to xc2xd in. diameter studs can weigh approximately 4xc2xd lb. A larger gun, weighing approximately 11 lb, can be used for welding ⅝ to 1xc2xc-in. diameter studs. Typical gun bodies are made of high-impact strength plastic. FIG. 2 is a schematic drawing illustrating a hand-held stud arc welding gun. Stud arc welding guns can also incorporate a means for causing the stud to plunge or move slowly as it enters the molten pool of metal at the completion of the weld. This cushioning effect reduces weld splatter considerably and also improves weld integrity.
As shown in FIG. 2, stud gun 18 has a fixed core 44 and a movable core 46. Fixed core 44 is fixed to the rear end of stud gun 18, while movable core 46 is aligned with fixed core 44 along gun axis 43, and is situated toward the distal end of stud gun 18. An air gap 52 exists between fixed core 44 and movable core 46 to enable movable core 46 to move along gun axis 43. Stud gun 18 also includes a solenoid 42, and clutch assembly 38, and a lifting rod 36 that cooperate to move movable core 46 during the welding process.
Stud gun 18 can also include a foot 34 disposed on a distal end of a leg 32 that extends from the main body of stud gun 18. Foot 34 surrounds stud 26 between ferrule 22 and chuck 20. Stud gun 18 can be coupled to the power/control unit trigger via a control cable 50 and a weld cable 48. A trigger switch 40 is used to initiate the arc welding process.
Stud arc welding systems with automatic feed are available with both portable and fixed welding guns. A hand-held gun 18xe2x80x2 with an automatic stud feed attachment 52 is depicted in FIG. 3. Typically, studs are automatically oriented in a parts feeder and transferred through a flexible feed tube into the welding gun chuck. A ferrule or arc shield is hand-loaded for each weld. For special applications, inert gas shielding or a semi-permanent ferrule is used to eliminate the loading of a ferrule for each weld. Using automatic feed systems such as this, welding rates in the range of 20 to 45 studs per minute can be obtained.
Capacitor discharge stud welding, because no ceramic ferrule is required, is suited for high-speed automatic stud feed applications. Portable capacitor discharge equipment with automatic stud feed is available for studs ranging from No. 6 through xc2xc in. diam. Studs are automatically oriented in a parts feeder and transferred through a flexible feed tube into the welding gun chuck. The automatic feed attachments add very little weight to the gun and do not encumber its use. Welding rates with portable equipment range up to 60 studs per minute on applications where stud location tolerances are such that no templating or only a loose-fitting templating is required.
Typically, in the arc welding method, larger studs (i.e., studs having a diameter greater than about xe2x85x9cxe2x80x3) are welded using a ceramic ferrule. The main reason for the ferrule is to control the shape of the weld puddle. In these systems, the studs and ferrules are loaded manually as it is very difficult to feed the brittle ceramic ferrules. Similarly, the remains of the ferrule need to be removed manually after the weld process. This results in a slow process, and tedious, labor intensive work.
Ferrules are required for the stud arc welding process except under highly specialized conditions. The ferrule surrounds the weld area and performs several important functions during the weld cycle, such as concentrating the heat of the arc in the weld area during the weld, reducing oxidation of the molten metal during welding by restricting passage of air into the weld area, and confining the molten metal to the weld area. The ferrule also protects the eyes of the operator from the arc; however, safety glasses with side shields and shade No. 3 filter lenses are recommended.
Two types of ferrules are used: expendable and semipermanent. The expendable ferrule has the broadest commercial use. It is composed of a ceramic material and breaks easily for removal. Because the expendable ferrule is designed for only one weld, it is much smaller, and its design, relative to venting and fillet cavity dimensions, can be optimized. Better fillet control and weld quality can be achieved with the expendable ferrule than with the semipermanent ferrule. Stud shape is not limited, because the ferrule does not have to slip over the stud shank of the welded stud for removal.
The semipermanent ferrule is seldom used and is suitable for special applications involving automatic stud feed systems in which fillet control is not important. The number of welds that can be obtained with a semipermanent ferrule varies considerably, depending on the stud diameter, weld setup, and weld rate, but is generally between 2500 and 7500. The ferrule fails because of the gradual erosion of the ferrule material by the molten metal, causing welds to become unacceptable.
FIGS. 4A-4E depict a conventional stud arc welding sequence in which a ferrule is used. The welding sequence begins by loading stud 26 into chuck 20 and a ferrule 22 into the ferrule holder. The relationship between stud 26 and ferrule 22 prior to positioning stud 26 on workpiece 30 is shown in FIG. 4A. Stud 26 protrudes beyond ferrule 22 (by a distance d known as a xe2x80x9cplungexe2x80x9d) to allow for stud burnoff and to enable stud 26 to plunge fully into the molten metal once the arcing time is completed. Stud 26 and ferrule 22 are then placed on a receiving surface of workpiece 30 as shown in FIG. 4B.
With stud 26 now flush with the face of ferrule 22, a mainspring in stud gun 18 (FIG. 2) is compressed. When trigger switch 40 is operated, solenoid coil 42 is energized, causing stud 26 to lift from workpiece 30 and create an arc 29 as shown in FIG. 4C. The heat from arc 29 causes both stud 26 and a portion of the receiving surface of workpiece 30 to melt. When the arc period, as preset and maintained by control unit 16, is completed, solenoid coil 42 is de-energized, and the weld current is automatically shut off. De-energizing solenoid coil 42 allows the mainspring in stud gun 18 to force stud 26 into the molten pool on workpiece 30 to complete the weld (see FIG. 4D). Stud gun 18 is then lifted from the welded stud, and ferrule 22 is removed. FIG. 4E shows stud 26 in place on workpiece 30 after the weld has been competed and ferrule 22 removed.
In robotic stud welding applications, a stud welding gun is attached to a robot which is programmed to position the gun to the desired weld location and to automatically produce stud welds, without the need for an operator. In a conventional automated stud welding system, system resource utility lines or cables are typically fed directly into the stud welding gun. For example, a stud feed tube, i.e., a hollow utility line or cable, may be fed directly into a stud welding gun to transfer a plurality of studs from a remote stud feeder to the stud welding gun. The remote stud feeder may provide air pressure to transport the studs through the feed tube. As the stud welding gun receives the flow of studs, the stud welding operation may be continuously performed. Another utility line or cable may also be fed directly into the stud welding gun for providing the gun with a weld current. Usually, this cable will extend from a remote power source/controller, providing the necessary current and its duration. Other utility lines may also extend directly into the gun, e.g., pneumatic lines for operating the gun, air or liquid lines for cleaning the weld surface, etc.
A detailed description of stud welding apparatus and methods is provided in xe2x80x9cStud Welding,xe2x80x9d American Society of Metals Handbook, Vol. 6.
Although robotic stud welding systems are known for welding smaller diameter studs, the use of ferrules in the arc welding of larger diameter studs has heretofore prevented the development of robotic systems for arc welding such studs. Thus, there is a need in the art for a robotic stud arc welding system capable of automatically arc welding larger diameter studs to a receiving surface without the use of a ferrule.
The present invention satisfies these needs in the art by providing systems for robotic arc welding of large studs (i.e., studs having a diameter of greater than about xe2x85x9c inch) to a workpiece using suitable assist gas without the use of a ferrule. Apparatus for arc welding a stud to a workpiece without the use of a ferrule comprises a stud arc welding gun having a spacer at a distal end thereof, and a chuck adapted to receive a stud having a diameter of more than about xe2x85x9c inch.
A stud feeding device is coupled to the gun for automatically loading studs into the gun. The stud feeding device determines whether a stud is loaded into the gun and, if no stud is loaded into the gun, automatically loads a stud into the gun. The stud feeding device can determine a proper orientation of the stud and load the stud into the chuck in the proper orientation. The stud feeding device can include a bowl feeder in which a plurality of studs is stored, and a stud feeding conduit that couples the stud gun with the bowl feeder, via which studs are transferred from the bowl feeder to the stud gun.
A gun positioning device, such as an arm of a robot, is coupled to the gun, and automatically positions the gun such that the spacer is against a first receiving area of the workpiece. A controller coupled to the stud gun initiates a stud arc welding process to arc weld the stud to the workpiece. The robot can be adapted to weld a plurality of studs by automatically moving the stud gun away from the first receiving area and positioning it at a second receiving area on the same or a different workpiece.